To increase the density of package structures, multiple dies may need to be packaged in a same package structure. To accommodate multiple dies, an interposer is typically used to bond dies thereon and the resulting structure including the interposer and the dies are treated as a single structure which may be bonded to a printed circuit board (PCB). It is desirable to form silicon interposers due to the fact that the technology for handling silicon substrates is mature. Further, features formed on silicon interposers may have a high density. However, due to various factors, such as the significant difference between the coefficient of thermal expansion (CTE) of silicon and the CTE of the PCB, it is difficult to bond a silicon interposer directly on a PCB without incurring reliability issues.
A solution for the above-discussed problem is to insert an organic substrate between the silicon interposer and the PCB. This solution, however, still suffers from other issues. Firstly, organic substrates have a high cost. Secondly, organic substrates are typically soft, and hence may not be able to provide enough protection needed by other package components. Thirdly, the signal loss in organic substrates is high, particularly when the signal has a high frequency.
Current approach is to investigate the use of glass as the interposer substrate due to higher resistivity and lower dielectric constant of glass compared with silicon. The substrates can also be etched deep into the material with high aspect ratios and this technology has been gaining in through-glass via fabrication. However, the substrates typically include mobile ions, such as alkaline ions, which diffuse into the conductive vias and induce mobile charges in the interposer. These induced charges will cause capacitance variations and inductance loss, degrading the integrity of further device processes.